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Calling C-DLL Again

rene041697
rene041697 asked
on
Medium Priority
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Last Modified: 2010-04-06
Help !!

I trie to call a function from a DLL called scom32. The DLL is written in MSVC++ 4.00 and i only have the headerfile.
I have converted the headerfile to delphi and try to call the funtion. But something goes wrong with i think passing the function arguments. When the function i call only has one argument then the call works, but when i have two or more arguments the call actualy changes arguments wich normally would't change.

Here is what i did:

-------------- C-header:

typedef struct tagPCDCONN
{
  int Port;
  int Protocol;
  int Baud;
  BYTE Sbusmode;
  BOOL bEPROM;
  BYTE TxMsg[270];
} PCDCONN;

typedef PCDCONN FAR *LPCONN;

int WINAPI PcdComOpen(LPCONN);
int WINAPI PcdRdVersion(LPCONN,LPSTR);

-------------- This is my DELPHI equivalent:

type
 PCDCONN = Record
   Port    :Integer;
   Protocol:Integer;
   Baud    :Integer;
   Sbusmode:Byte;
   bEPROM  :Longbool;
   TxMsg   :Array[0..269] of Byte;
 end;

var

 LPCONN : ^PCDCONN;
 Version: Array [0..127] of AnsiChar;
 Status : Integer;

function PcdComOpen(LPCONNEC:PCDCONN):Integer;
         far; pascal; external 'SCOM32';
function PcdRdVersion(LPCONNEC:PCDCONN;Version:PChar):
         Integer; far; pascal; external 'SCOM32';
                     
-------------- And so i call the functions --------
 begin
  getmem(LPCONN,SizeOf(PCDCONN);
  PCDCONN^.Port:=0;
   etc.etc.
  status:=PcdComOpen(PCDCONN^); {1}
  status:=PcdRdVersion(PCDCONN^,Version); {2}
  freemem(LPCONN,SizeOf(PCDCONN);
 end;

When i check what is returned in the status then i can only say it works correct (in statement {1} as well in {2}). But in statement {2} the result in 'version' is wrong and 'PDCONN' value's are destroyed. Only the first two fields are not correct anymore, so i suspect it has something to do with memory allocation or with passing the function arguments.
I tried all the other calling conventions (stdcall,cdecl etc.) but the give a access violation error. Pascal is the
only calling method wich gives the result described above.
I have searched for two days but can't seem to find out what the problem is.

Is there anyone who can help me !!!

Kind Regards,

Rene Reuscher, ANTONIUS Vessel heads
Comment
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Commented:
>-------------- This is my DELPHI equivalent:
>type
> PCDCONN = Record
>   Port :Integer;
>   Protocol:Integer;
>   Baud :Integer;
>   Sbusmode:Byte;
>   bEPROM :Longbool;
>   TxMsg :Array[0..269] of Byte;
> end;

This basically looks okay, though I don't think a C "bool" translates to a Delphi longbool, wordbool maybe?

>var
> LPCONN : ^PCDCONN;

I wouldn't bother with this, see below

>Version: Array [0..127] of AnsiChar;
>Status : Integer;

>function PcdComOpen(LPCONNEC:PCDCONN):Integer;
>    far; pascal; external 'SCOM32';

I'd change this to:
function PcdComOpen( var PCD: PCDCONN );
  far; StdCall; external 'SCOM32' ;

>function PcdRdVersion(LPCONNEC:PCDCONN;Version:PChar):
>    Integer; far; pascal; external 'SCOM32';

Similarly:
function PcdRdVersion( var PCD: PCDCONN Version: PChar );
  far; StdCall; external 'SCOM32' ;


>-------------- And so i call the functions --------
> begin
>  getmem(LPCONN,SizeOf(PCDCONN);
>  PCDCONN^.Port:=0;

I'm assuming you've got your variable/type names mixed up a bit here, didn't you mean LPCONN^.Port ?

>   etc.etc.
>  status:=PcdComOpen(PCDCONN^); {1}
>  status:=PcdRdVersion(PCDCONN^,Version); {2}
>  freemem(LPCONN,SizeOf(PCDCONN);
> end;


Anyways, try:
var
  LPCONN : PCDCONN ;
begin
  LPCONN.Port := 0 ;
  ...
  Status := PcdCommOpen( LPCONN ) ;
  Status := PcdRdVersion( LPCONN, Version );
end ;

HTH

slp

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Author

Commented:
Great !! it works. The biggest problem i seem to have was the
missing 'var' statement in my function call because the function
changes my arguments. (Stupid,stupid of me). But then i find it strange that i didn't got an error message when i used the 'pascal' calling method.
Anyway, Thanks A lot!!!!
slp

Commented:
Actually not so strange, the "pascal;" calling directive is (I believe) largely functionally equivalent to the "StdCall;" directive.  As I understand it, StdCall is the D2 version that is supposed to remain compatible with any changes in the Windows DLL conventions whereas pascal matches the original spec for backward compatibility; since the convention hasn't changed significantly yet they are the same at the moment.

slp





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